This might not be a problem with the RX or servo.

Your engine is leaning out a bit in the air, so you may get a good response when the plane is sitting on the ground, but in the air, it is hesitating as it runs up.

You will perceive this as a slow throttle response.

Your best bet is to tune the engine with the tank purposely low, then check the tuning by pointing the plane nose high and running the throttle up and down.

This puts the plane in a "worst case" condition.

Get it set up properly for worst case, and it should run better most of the other time.